Do MRI scans show signs that someone may have an intellectual disability (otherwise known as mental retardation)?

No. MRI scans show views of the anantomy of the brain. It cannot tell intellectusl ability, but it can detect abnormalities of brain structure that can help identify specific patterns related to mental retardation, and areas of abnormal development or damage.
No. An MRI can identify normal,abnormally formed or damaged areas, but that does not translate directly to abnormal function. A person with intellectual impairment may have a totally normal MRI, Someone with an abnormal MRI may have some impairments but be intellectually normal.